As progress toward higher frequency performance, higher miniaturization and higher device integration takes place in bipolar transistor technology, the design of the transistor so as to improve one parameter frequently results in the deterioration of another parameter.
This is particularly the case in heterojunction bipolar transistors that are fabricated in arrays in superimposed layers in compound semiconductor materials where desirable device structural features are frequently limited both in attainability and in reproducibility by the processing techniques involved in the fabrication. In structures of this type limitations in the ability to achieve precision in the processing operations of erosion of materials and positioning of deposited materials necessitates tradeoffs due to tolerances in accommodating these limitations to interfere with performance, miniaturization and reproducibility. The structure of the device and the materials from which it is made are of importance in the ability to fabricate it reproducibly.